Non-aqueous electrolyte secondary battery

Abstract

The present invention provides a nonaqueous electrolyte secondary battery which uses, as a positive electrode active material, a lithium transition metal composite oxide that has a high Ni content and a low Co content, and which is suppressed in a capacity decrease that is associated with charging and discharging. A nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure is provided with a positive electrode, a negative electrode and a nonaqueous electrolyte; the positive electrode contains a lithium transition metal composite oxide that contains Ni, Nb and M (M is at least one element selected from among Ca and Sr), said lithium transition metal composite oxide optionally containing Co; the contents of Ni, Nb, M and Co in the lithium transition metal composite oxide are respectively within specific ranges; the negative electrode comprises a negative electrode mixture layer that contains a negative electrode active material, and a coating film that is formed on the surface of the negative electrode mixture layer, while containing Nb and M; and the contents of Nb and M in the negative electrode are respectively within specific ranges.

Description

[Technical Field]

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery that includes a lithium transition metal composite oxide containing Ni as a positive electrode active material. [Background technology]

[0002] In recent years, lithium transition metal composite oxides with a high Ni content have been attracting attention as a positive electrode active material with a high energy density. For example, Patent Document 1 discloses a compound represented by the general formula Li x Ni 1-y-z-v-w Co y Al z M 1 v M 2 w The lithium transition metal composite oxide is represented by the formula: O2, wherein the element M 1 is at least one element selected from Mn, Ti, Y, Nb, Mo, and W, and element M 2 Patent Document 2 discloses a positive electrode active material in which the elements are at least Mg and Ca. Patent Document 2 also discloses a lithium transition metal composite oxide containing Ni, Mn, and Co, which composite oxide contains at least one element selected from Mo, W, Nb, Ta, and Re. However, since Co is expensive, there is a demand to reduce the amount of Co used. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-310181 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-289726 Summary of the Invention [Problem to be solved by the invention]

[0004] However, when the Co content is reduced in a lithium transition metal composite oxide with a high Ni content, the structure of the composite oxide becomes unstable, making it more likely for side reactions with the electrolyte to occur on the particle surface of the composite oxide. This is thought to result in the generation of large amounts of electrolyte decomposition products, which then form a coating of the decomposition products on the negative electrode surface, thereby deteriorating the charge / discharge cycle characteristics of the battery. The technologies disclosed in Patent Documents 1 and 2 still have room for improvement in terms of cycle characteristics.

[0005] An object of the present disclosure is to suppress a decrease in capacity due to charge and discharge in a nonaqueous electrolyte secondary battery that uses a lithium transition metal composite oxide with a high Ni content and a low Co content as a positive electrode active material. [Means for solving the problem]

[0006] A non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode contains a lithium transition metal composite oxide containing Ni, Nb, M (M is at least one element selected from Ca and Sr), and Co as an optional component. In the lithium transition metal composite oxide, the Ni content is 80 mol % or more relative to the total number of moles of metal elements excluding Li, the Nb content is 0.35 mol % or less relative to the total number of moles of metal elements excluding Li, and the M content is L the content of Co is 1 mol % or less relative to the total number of moles of metal elements excluding Li, and the content of Co is 5 mol % or less relative to the total number of moles of metal elements excluding Li; the negative electrode has a negative electrode composite layer containing a negative electrode active material, and a coating containing Nb and M formed on the surface of the negative electrode composite layer, and in the negative electrode, the content of Nb is 10 ppm to 3000 ppm relative to the total mass of the negative electrode composite layer and the coating, and the content of M is 10 ppm to 3000 ppm relative to the total mass of the negative electrode composite layer and the coating. [Effects of the Invention]

[0007] According to one aspect of the present disclosure, a nonaqueous electrolyte secondary battery using a lithium transition metal composite oxide with a high Ni content and a low Co content as a positive electrode active material can suppress a decrease in capacity due to charge and discharge. The nonaqueous electrolyte secondary battery according to the present disclosure has excellent charge and discharge cycle characteristics. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment; DETAILED DESCRIPTION OF THE INVENTION

[0009] As described above, lithium transition metal composite oxides with a high Ni content and a low Co content have an unstable structure, which is likely to cause side reactions with the electrolyte on the particle surface, resulting in a deterioration in the charge-discharge cycle characteristics of the battery. The inventors have discovered that by using a lithium transition metal composite oxide containing predetermined amounts of Nb and M (M is at least one element selected from Ca and Sr), the formation and erosion of a structurally deteriorated layer on the surface of the lithium transition metal composite oxide due to reactions with the electrolyte, etc., can be further suppressed at the positive electrode, while a high-quality coating containing Nb and M derived from the positive electrode can be formed on the surface of the negative electrode, thereby improving the charge-discharge cycle characteristics.

[0010] When a conventional lithium transition metal composite oxide is used, a coating containing a large amount of Li is likely to be formed on the negative electrode surface due to decomposition products of the electrolyte, and this coating is thought to be one of the factors that deteriorate the charge-discharge cycle characteristics. In the nonaqueous electrolyte secondary battery according to the present disclosure, the formation of this coating is suppressed, and instead a high-quality coating containing Nb and M is formed on the negative electrode surface, which is thought to have significantly improved the charge-discharge cycle characteristics.

[0011] In this specification, the expression "numerical value (A) to numerical value (B)" means that the value is equal to or greater than numerical value (A) and equal to or less than numerical value (B).

[0012] An example of an embodiment of a positive electrode active material for a nonaqueous electrolyte secondary battery according to the present disclosure and a nonaqueous electrolyte secondary battery using the positive electrode active material will be described in detail below. Hereinafter, a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom will be exemplified. However, the outer can is not limited to a cylindrical outer can and may be, for example, a rectangular outer can or an outer can made of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode assembly may be a laminated electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0013] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As illustrated in FIG. 1, the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte, and an outer can 16 that accommodates the electrode assembly 14 and the electrolyte. The electrode assembly 14 includes a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The outer can 16 is a cylindrical metal container that is open on one axial side and has a bottom, and the opening of the outer can 16 is closed by a sealing member 17. Hereinafter, for convenience of explanation, the sealing member 17 side of the battery is referred to as the top, and the bottom side of the outer can 16 is referred to as the bottom.

[0014] The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with halogen atoms such as fluorine. The electrolyte salt may be, for example, a lithium salt such as LiPF6. The electrolyte is not limited to a liquid electrolyte, and may be a solid electrolyte using a gel polymer or the like.

[0015] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all strip-shaped, long bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the longitudinal direction and width direction (short direction). The two separators 13 are formed to be at least slightly larger than the positive electrode 11 and are arranged, for example, to sandwich the positive electrode 11. The electrode assembly 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0016] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1 , the positive electrode lead 20 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the outer can 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

[0017] A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure airtightness inside the battery. The outer can 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the outer can 16 by the grooved portion 22 and the open end of the outer can 16 that is crimped to the sealing body 17.

[0018] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0019] The positive electrode 11, negative electrode 12, and separator 13 that constitute the electrode assembly 14, and in particular the positive electrode active material that constitutes the positive electrode 11, will be described in detail below.

[0020] [Positive electrode] The positive electrode 11 has a positive electrode core and a positive electrode composite layer provided on the surface of the positive electrode core. The positive electrode core can be a foil of a metal such as aluminum that is stable within the potential range of the positive electrode 11, or a film with such a metal disposed on the surface. The positive electrode composite layer contains a positive electrode active material, a binder, and a conductive material, and is preferably provided on both sides of the positive electrode core except for the portion to which the positive electrode lead 20 is connected. The positive electrode 11 can be produced, for example, by applying a positive electrode composite slurry containing a positive electrode active material, a binder, a conductive material, etc. to the surface of the positive electrode core, drying the coating, and then compressing it to form a positive electrode composite layer on both sides of the positive electrode core.

[0021] Examples of conductive materials contained in the positive electrode mixture layer include carbon materials such as carbon black, acetylene black, ketjen black, graphite, carbon nanotubes (CNT), and graphene. Examples of binders contained in the positive electrode mixture layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. These resins may be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or its salts, polyethylene oxide (PEO), and the like.

[0022] The positive electrode 11 contains a lithium transition metal composite oxide containing Ni, Nb, M (M is at least one element selected from Ca and Sr), and an optional component, Co. Hereinafter, for convenience of explanation, this lithium transition metal composite oxide will be referred to as "composite oxide (Z)." The composite oxide (Z) functions as a positive electrode active material. The positive electrode active material may contain the composite oxide (Z) as a main component and may be substantially composed of the composite oxide (Z) alone. Note that the positive electrode active material may contain a composite oxide other than the composite oxide (Z) or other compounds, as long as the object of the present disclosure is not impaired.

[0023] The composite oxide (Z) may have a layered structure, for example, a layered structure belonging to the space group R-3m or a layered structure belonging to the space group C2 / m.

[0024] The composite oxide (Z) is, for example, a secondary particle formed by the aggregation of multiple primary particles. The particle size of the primary particles is generally 0.05 μm to 1 μm. The volume-based median diameter (D50) of the composite oxide (Z) is, for example, 3 μm to 30 μm, preferably 5 μm to 25 μm. D50 refers to the particle size at which the cumulative frequency of the smallest particle size in the volume-based particle size distribution is 50%, and is also called the median diameter. The particle size distribution of the composite oxide (Z) can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II, manufactured by Microtrack Bell Corporation) using water as a dispersion medium.

[0025] The composite oxide (Z) contains 80 mol % or more of Ni relative to the total number of moles of metal elements excluding Li. By making the Ni content 80 mol % or more, a battery with high energy density can be obtained. The Ni content may be 85 mol % or more, or may be 90 mol % or more, relative to the total number of moles of metal elements excluding Li. Furthermore, the Ni contained in the composite oxide (Z) is a Ni source for the coating formed on the surface of the negative electrode; a portion of the Ni is eluted and deposited on the surface of the negative electrode during charge and discharge, and is contained in the coating of the negative electrode.

[0026] When the composite oxide (Z) contains Co, the Co content is 5 mol % or less based on the total number of moles of metal elements excluding Li. Since Co is expensive, it is preferable to use a small amount of Co. Preferably, the composite oxide (Z) contains 2 mol % or less of Co based on the total number of moles of metal elements excluding Li, or contains substantially no Co. "Substantially no Co" means that no Co is contained at all, or that Co is present as an impurity (that is, Co is present to such an extent that it cannot be accurately quantified).

[0027] The Nb content in the composite oxide (Z) is 0.35 mol % or less, and preferably 0.30 mol % or less, relative to the total number of moles of metal elements excluding Li. If the Nb content exceeds 0.35 mol %, the resistance increases and the charge capacity decreases. There is no particular lower limit for the Nb content in the composite oxide (Z). If the composite oxide (Z) contains Nb together with M, which will be described later, the synergistic effect of Nb and M can improve the charge-discharge cycle characteristics. Furthermore, the Nb content is preferably 0.05 mol % or more. In this case, the improvement effect on the charge-discharge cycle characteristics is more pronounced.

[0028] The content of M (M is at least one element selected from Ca and Sr) in the composite oxide (Z) is 0.35 mol % or less, and preferably 0.30 mol % or less, relative to the total number of moles of metal elements excluding Li. If the content of M exceeds 0.35 mol %, the resistance increases and the charge capacity decreases. There is no particular lower limit for the content of M in the composite oxide (Z). If the composite oxide (Z) contains M together with the above-mentioned Nb, the synergistic effect of M and Nb can improve the charge-discharge cycle characteristics. Furthermore, the content of M is preferably 0.05 mol % or more. In this case, the effect of improving the charge-discharge cycle characteristics becomes more pronounced.

[0029] The Nb and M contained in the composite oxide (Z) suppress the formation and erosion of a structurally deteriorated layer on the surface of the lithium transition metal composite oxide at the positive electrode due to, for example, a reaction with the electrolyte. The Nb and M contained in the composite oxide (Z) are Nb and M sources for the coating formed on the surface of the negative electrode. Part of them dissolve and deposit on the negative electrode surface during charge and discharge, becoming incorporated into the coating on the negative electrode. The state of Nb in the composite oxide (Z) is not particularly limited, but it is preferable that Nb form a solid solution with other metal elements such as Ni. It is more preferable that 80% or more of the Nb contained in the composite oxide (Z) is solid-solved in the composite oxide, and it is particularly preferable that substantially all of the Nb is solid-solved. The amount of Nb in solid solution can be confirmed by energy dispersive X-ray spectroscopy (EDS). The state of M contained in the composite oxide (Z) is not particularly limited, but it is preferable that it is present on the surfaces of the primary and secondary particles of the composite oxide (Z).

[0030] The composite oxide (Z) may contain a metal element other than Li, Ni, Nb, M, and Co. Examples of such metal elements include Mn, Al, Zr, B, Mg, Fe, Cu, Zn, Sn, Na, K, Ba, W, Mo, and Si. Among these, the composite oxide (Z) preferably contains at least one of Mn and Al. When the composite oxide (Z) contains Mn, the content of Mn is preferably 1 to 10 mol% based on the total number of moles of the metal elements excluding Li. When the composite oxide (Z) contains Al, the content of Al is preferably 1 to 10 mol% based on the total number of moles of the metal elements excluding Li.

[0031] It is preferable that the composite oxide (Z) further contains W. This can further improve the charge-discharge cycle characteristics. The W contained in the composite oxide (Z) suppresses the formation and erosion of a structurally deteriorated layer on the surface of the lithium transition metal composite oxide at the positive electrode due to reaction with the electrolyte, etc. Furthermore, the W contained in the composite oxide (Z) is a W source for the coating formed on the surface of the negative electrode. A portion of the W is eluted and deposited on the surface of the negative electrode during charge and discharge, and is contained in the coating on the negative electrode. The W content in the composite oxide (Z) is preferably 0.5 mol % or less, more preferably 0.4 mol % or less, based on the total number of moles of metal elements excluding Li. The lower limit of the W content in the composite oxide (Z) is not particularly limited, but may be, for example, 0.01 mol % or more, or 0.05 mol % or more. The state of W in the composite oxide (Z) is not particularly limited, and may be, for example, present on the surfaces of primary and secondary particles of the composite oxide (Z), or may be solid-solved in the composite oxide (Z).

[0032] An example of a suitable composite oxide (Z) is a compound represented by the general formula Li a Ni b Co c Al d Mn e Nb f M g O hIt is a composite oxide represented by (where 0.8 ≦ a ≦ 1.2, 0.80 ≦ b < 1, 0 ≦ c ≦ 0.05, 0 ≦ d ≦ 0.10, 0 ≦ e ≦ 0.10, 0 < f ≦ 0.0035, 0 < g ≦ 0.0035, 1 ≦ h ≦ 2, and b + c + d + e + f + g = 1). Preferably, 0.85 ≦ b < 1, 0 ≦ c ≦ 0.02, 0 < f ≦ 0.0030, 0 < g ≦ 0.0030, and more preferably 0.85 ≦ b < 0.95, 0 ≦ c ≦ 0.01, 0.0005 ≦ f ≦ 0.0030, 0.0005 ≦ g ≦ 0.0030.

[0033] The content rate of the elements constituting the composite oxide (Z) can be measured by an inductively coupled plasma atomic emission spectrometer (ICP - AES), an electron probe microanalyzer (EPMA), an energy - dispersive X - ray analyzer (EDX), or the like.

[0034] The composite oxide (Z) can be synthesized, for example, by mixing a transition metal oxide containing Ni, Al, Mn, etc., a Nb raw material, an M raw material, and a Li raw material such as lithium hydroxide (LiOH) and firing them. Also, after mixing a transition metal oxide containing Ni, Al, Mn, etc., a Nb raw material, and an M raw material and firing them to synthesize a composite oxide containing Ni, Nb, and M, the composite oxide (Z) may be synthesized by adding a Li raw material and firing again. The firing is carried out, for example, at a temperature of 600°C to 800°C in an oxygen atmosphere. Examples of the Nb raw material include Nb2O5, Nb2O5·nH2O, LiNbO3, NbCl5, etc. Examples of the M raw material include Ca(OH)2, CaO, CaCO3, CaSO4, Ca(NO3)2, Sr(OH)2, Sr(OH)2·8H2O, SrO, SrCO3, SrSO4, Sr(NO3)2, etc.

[0035] The composite oxide (Z) obtained in the above step may be washed with water, mixed with a W raw material, and heat-treated to incorporate W into the composite oxide (Z). Alternatively, the composite oxide (Z) obtained in the above step may be mixed with a W raw material, washed with water, and heat-treated to incorporate W into the composite oxide (Z). The heat treatment may be performed, for example, in a vacuum at a temperature of 150°C to 600°C. Examples of W raw materials include tungsten oxide (WO3), lithium tungstate (Li2WO4, Li4WO5, Li6W2O9), etc.

[0036] [Negative electrode] The negative electrode 12 has a negative electrode core and a negative electrode composite layer provided on the surface of the negative electrode core. The negative electrode core can be a foil of a metal such as copper that is stable within the potential range of the negative electrode 12, or a film with such a metal disposed on the surface. The negative electrode composite layer contains a negative electrode active material and a binder, and is preferably provided on both sides of the negative electrode core except for the portion to which the negative electrode lead 21 is connected. The negative electrode 12 can be produced, for example, by applying a negative electrode composite slurry containing a negative electrode active material and a binder to the surface of the negative electrode core, drying the coating, and then compressing it to form a negative electrode composite layer on both sides of the negative electrode core.

[0037] The negative electrode mixture layer contains, as the negative electrode active material, for example, a carbon-based active material that reversibly absorbs and releases lithium ions. Suitable carbon-based active materials include natural graphite such as flake graphite, lump graphite, and amorphous graphite, and artificial graphite such as lump artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). The negative electrode active material may be a Si-based active material composed of at least one of Si and a Si-containing compound, or a combination of a carbon-based active material and a Si-based active material.

[0038] The binder contained in the negative electrode mixture layer may be, as in the case of the positive electrode 11, a fluororesin, PAN, polyimide, acrylic resin, polyolefin, or the like, but is preferably styrene-butadiene rubber (SBR). The negative electrode mixture layer preferably further contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like. Among these, it is preferable to use SBR in combination with CMC or a salt thereof, or PAA or a salt thereof.

[0039] The negative electrode 12 has a coating (hereinafter sometimes referred to as the "negative electrode coating") containing Nb and M (M is at least one element selected from Ca and Sr) formed on the surface of the negative electrode composite layer. The negative electrode coating is thought to be formed when Nb and M in the complex oxide (Z) eluted during charge and discharge are deposited on the surface of the negative electrode composite layer. That is, the negative electrode coating contains Nb and M derived from the complex oxide (Z). The negative electrode coating is formed, for example, by 10 or fewer charge and discharge cycles. By using the complex oxide (Z) containing predetermined amounts of Nb and M and forming a high-quality coating containing Nb and M derived from the positive electrode on the negative electrode surface, capacity loss due to charge and discharge is suppressed and good cycle characteristics are obtained. The presence of the negative electrode coating can be confirmed, for example, by X-ray photoelectron spectroscopy (XPS).

[0040] The Nb content in the negative electrode is 10 ppm to 3000 ppm relative to the total mass of the negative electrode composite layer and coating. If the Nb content is less than 10 ppm or more than 3000 ppm, the effect of improving charge-discharge cycle characteristics cannot be obtained. The Nb content in the negative electrode can be controlled by the composition of the composite oxide (Z), particularly the Nb content, as well as the charge-discharge conditions. For example, increasing the end-of-charge voltage and deepening the depth of discharge tends to increase the Nb content in the negative electrode.

[0041] The Nb content in the negative electrode relative to the total mass of the negative electrode composite layer and coating can be calculated by the following method. The M content, Ni content, and W content in the negative electrode, which will be described later, can also be calculated by the same method. (1) Ion-exchanged water is added to the negative electrode 12, the negative electrode composite layer and the coating are removed from the negative electrode core, and the weights of the negative electrode composite layer and the coating are measured. (2) Aqua regia and hydrofluoric acid are added to the separated negative electrode composite layer and coating, and the mixture is heated and dissolved. Insoluble matters such as carbon are filtered off to prepare an aqueous solution. The aqueous solution is adjusted to a constant volume with ion-exchanged water, and the Nb concentration is measured by ICP-AES. The result is taken as the Nb content in the negative electrode. (3) The Nb content in the negative electrode measured in (2) was divided by the weight of the negative electrode mixture layer and coating measured in (1) to obtain the Nb content in the negative electrode.

[0042] The content of M in the negative electrode is 10 ppm to 3000 ppm relative to the total mass of the negative electrode composite layer and the coating. If the content of M is less than 10 ppm or more than 3000 ppm, the effect of improving the charge-discharge cycle characteristics cannot be obtained. The content of M in the negative electrode can be controlled by the composition of the composite oxide (Z), particularly the content of M, as well as the charge-discharge conditions. For example, if the end-of-charge voltage is increased and the depth of discharge is deepened, the content of M in the negative electrode tends to increase.

[0043] The negative electrode coating may further contain Ni. It is believed that Ni in the composite oxide (Z) eluted during charge and discharge is deposited on the surface of the negative electrode mixture layer together with Nb and M to form the negative electrode coating. That is, the negative electrode coating contains Ni derived from the composite oxide (Z).

[0044] In the negative electrode, it is preferable that the mass ratio of the Nb content to the Ni content (Nb / Ni) is 0.3 to 5, and the mass ratio of the M content to the Ni content (M / Ni) is 0.3 to 10. If the Nb / Ni ratio and the M / Ni ratio are within the above ranges, the effect of improving cycle characteristics can be enhanced. The Nb / Ni ratio and the M / Ni ratio can be controlled by the composition of the composite oxide (Z), particularly the ratio of the Nb, M, and Ni contents, as well as the charge / discharge conditions.

[0045] When the lithium transition metal composite oxide contained in the positive electrode contains W, the W content in the negative electrode is preferably 10 ppm to 3000 ppm relative to the total mass of the negative electrode composite layer and the coating, and the mass ratio of the W content to the Ni content (W / Ni) is preferably 0.3 to 5. This can further enhance the effect of improving cycle characteristics. The negative electrode may also contain metal elements other than Nb, M, W, and Ni. The negative electrode contains, for example, metal elements such as Nb, M, W, and Ni, and organic substances that are decomposition products of the electrolyte.

[0046] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may have either a single-layer structure or a laminated structure. A heat-resistant layer or the like may be formed on the surface of the separator. [Example]

[0047] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0048] Example 1 [Synthesis of lithium transition metal composite oxide (positive electrode active material)] General formula Ni 0.93 Al 0.05 Mn 0.02 The composite oxide was mixed with niobium hydroxide (Nb2O5·nH2O) and calcium hydroxide (Ca(OH)2) so that the Nb content was 0.35 mol% and the Ca content was 0.3 mol% relative to the total amount of Ni, Al, and Mn in the composite oxide, represented by O2. Lithium hydroxide (LiOH) was then added so that the molar ratio of the total amount of Ni, Al, Mn, Nb, and Ca to Li was 1:1.03. The mixture was placed in a firing furnace and sintered under an oxygen flow (10 cm) with an oxygen concentration of 95%. 3The mixture was calcined from room temperature to 650°C at a temperature increase rate of 2.0°C / min (flow rate of 2 mL / min per kg of mixture and 5 L / min per kg of mixture). The mixture was then calcined from 650°C to 715°C at a temperature increase rate of 0.5°C / min, and the calcined product was washed with water to obtain a lithium transition metal composite oxide. The results of ICP-AES analysis of the lithium transition metal composite oxide are shown in Table 1.

[0049] [Preparation of positive electrode] The lithium transition metal composite oxide was used as the positive electrode active material. The positive electrode active material, acetylene black, and polyvinylidene fluoride (PVdF) were mixed in a solids mass ratio of 95:3:2, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added. The mixture was then kneaded to prepare a positive electrode composite slurry. The positive electrode composite slurry was applied to both sides of a positive electrode core made of aluminum foil, and the coating was dried. The coating was then rolled using a roller and cut to a predetermined electrode size to obtain a positive electrode with a positive electrode composite layer formed on both sides of the positive electrode core. An exposed portion was provided on a portion of the positive electrode, exposing the surface of the positive electrode core.

[0050] [Preparation of negative electrode] Natural graphite was used as the negative electrode active material. The negative electrode active material, carboxymethyl cellulose sodium (CMC-Na), and styrene-butadiene rubber (SBR) were mixed in an aqueous solution at a solids mass ratio of 100:1:1 to prepare a negative electrode composite slurry. The negative electrode composite slurry was applied to both sides of a negative electrode core made of copper foil, and the coating was dried. The coating was then rolled using a roller and cut to a predetermined electrode size to obtain a negative electrode with a negative electrode composite layer formed on both sides of the negative electrode core. An exposed portion was provided in part of the negative electrode, exposing the surface of the negative electrode core.

[0051] [Preparation of non-aqueous electrolyte] A non-aqueous electrolyte solution was prepared by dissolving lithium hexafluorophosphate (LiPF6) at a concentration of 1.2 mol / L in a mixed solvent of ethylene carbonate (EC), methyl ethyl carbonate (MEC), and dimethyl carbonate (DMC) in a volume ratio of 3:3:4.

[0052] [Preparation of test cell (non-aqueous electrolyte secondary battery)] An aluminum lead was attached to the exposed portion of the positive electrode, and a nickel lead was attached to the exposed portion of the negative electrode, and the positive and negative electrodes were spirally wound with a polyolefin separator interposed therebetween, and then pressed radially to produce a flat wound electrode assembly. This electrode assembly was housed in an exterior body made of an aluminum laminate sheet, and the nonaqueous electrolyte solution was poured into it. The opening of the exterior body was then sealed to obtain a test cell.

[0053] [Evaluation of capacity retention rate after cycle test] The test cell was charged at a constant current of 0.5 It at a temperature of 25°C until the battery voltage reached 4.1 V, and then charged at a constant voltage until the current value reached 1 / 50 It at 4.1 V. It was then discharged at a constant current of 0.5 It until the battery voltage reached 2.85 V. This charge / discharge cycle was repeated 100 times. The discharge capacity at the first cycle and the discharge capacity at the 100th cycle of the cycle test were determined, and the capacity retention rate was calculated using the following formula: Capacity retention rate (%) = (100th cycle discharge capacity ÷ 1st cycle discharge capacity) × 100

[0054] <Example 2> In the synthesis of the positive electrode active material, the general formula Ni 0.92 Al 0.05 Mn 0.03 A test cell was fabricated in the same manner as in Example 1, except that a composite oxide represented by the formula O2 was used and mixed with Nb2O5·nH2O and strontium hydroxide (Sr(OH)2) so that the Nb content was 0.25 mol % and the Sr content was 0.1 mol %. The performance of the test cell was evaluated.

[0055] Example 3 A test cell was prepared in the same manner as in Example 2, except that in the synthesis of the positive electrode active material, the composite oxide, Nb2O5·nH2O, and Sr(OH)2 were mixed so that the Sr content was 0.3 mol%. The performance of the test cell was evaluated.

[0056] Example 4 A test cell was prepared in the same manner as in Example 2, except that in the synthesis of the positive electrode active material, the composite oxide, Nb2O5·nH2O, and Ca(OH)2 were mixed so that the Ca content was 0.1 mol%. The performance of the test cell was evaluated.

[0057] <Example 5> A test cell was prepared in the same manner as in Example 2, except that in the synthesis of the positive electrode active material, the composite oxide, Nb2O5·nH2O, and Ca(OH)2 were mixed so that the Ca content was 0.2 mol%. The performance of the test cell was evaluated.

[0058] Example 6 A test cell was prepared in the same manner as in Example 2, except that in the synthesis of the positive electrode active material, the composite oxide, Nb2O5·nH2O, Ca(OH)2, and Sr(OH)2 were mixed so that the Ca content was 0.1 mol% and the Sr content was 0.5 mol%. The performance of the test cell was evaluated.

[0059] Example 7 In the synthesis of the positive electrode active material, the general formula Ni 0.91 Al 0.05 Mn 0.04 A test cell was fabricated in the same manner as in Example 1, except that a composite oxide represented by the formula O2 was used and mixed with Nb2O5·nH2O and strontium hydroxide (Sr(OH)2) so that the Nb content was 0.2 mol % and the Sr content was 0.1 mol %. The performance of the test cell was evaluated.

[0060] Example 8 A test cell was prepared in the same manner as in Example 7, except that in the synthesis of the positive electrode active material, the composite oxide, Nb2O5·nH2O, and Ca(OH)2 were mixed so that the Ca content was 0.2 mol%. The performance of the test cell was evaluated.

[0061] Example 9 In the synthesis of the positive electrode active material, the general formula Ni 0.88 Co 0.01 Al 0.05 Mn 0.06A test cell was fabricated in the same manner as in Example 1, except that a composite oxide represented by the formula O2 was used and mixed with Nb2O5·nH2O and strontium hydroxide (Sr(OH)2) so that the Nb content was 0.1 mol % and the Sr content was 0.3 mol %. The performance of the test cell was evaluated.

[0062] Example 10 In the synthesis of the positive electrode active material, the general formula Ni 0.85 Al 0.05 Mn 0.10 A test cell was fabricated in the same manner as in Example 1, except that a composite oxide represented by formula (I) was used and mixed with NbO·nHO and Ca(OH) so that the Nb content was 0.2 mol % and the Ca content was 0.2 mol %.

[0063] Example 11 Test cells were prepared in the same manner as in Example 2, and their performance was evaluated, except that in the synthesis of the positive electrode active material, the composite oxide, NbO·nHO, and Sr(OH) were mixed and fired under an oxygen stream so that the Sr content was 0.3 mol %, and the fired material was washed with water, mixed with WO, and dried so that the W content was 0.4 mol %.

[0064] Example 12 A test cell was prepared in the same manner as in Example 11, except that in synthesizing the positive electrode active material, the composite oxide, Nb2O5·nH2O, Ca(OH)2, and WO3 were mixed so that the Ca content was 0.5 mol% and the W content was 0.2 mol%. The performance of the test cell was evaluated.

[0065] Example 13 A test cell was prepared in the same manner as in Example 11, except that in synthesizing the positive electrode active material, the composite oxide, Nb2O5·nH2O, Sr(OH)2, Ca(OH)2, and WO3 were mixed so that the Sr content was 0.1 mol %, the Ca content was 0.5 mol %, and the W content was 0.2 mol %.

[0066] <Comparative Example 1> A test cell was prepared in the same manner as in Example 1, except that Nb2O5·nH2O and Ca(OH)2 were not added in the synthesis of the positive electrode active material, and the performance was evaluated.

[0067] <Comparative Example 2> A test cell was fabricated in the same manner as in Example 2, except that Nb2O5·nH2O and Sr(OH)2 were not added in the synthesis of the positive electrode active material, and the performance was evaluated.

[0068] <Comparative Example 3> A test cell was produced in the same manner as in Example 2, except that Sr(OH)2 was not added in the synthesis of the positive electrode active material, and performance evaluation was carried out.

[0069] <Comparative Example 4> A test cell was prepared in the same manner as in Example 2, except that Nb2O5·nH2O was not added in the synthesis of the positive electrode active material, and the performance was evaluated.

[0070] <Comparative Example 5> A test cell was prepared in the same manner as in Example 7, except that Nb2O5·nH2O and Sr(OH)2 were not added in the synthesis of the positive electrode active material, and the performance was evaluated.

[0071] <Comparative Example 6> A test cell was prepared in the same manner as in Example 9, except that Nb2O5·nH2O and Sr(OH)2 were not added in the synthesis of the positive electrode active material, and the performance was evaluated.

[0072] <Comparative Example 7> A test cell was prepared in the same manner as in Example 10, except that Nb2O5·nH2O and Ca(OH)2 were not added in the synthesis of the positive electrode active material, and the performance was evaluated.

[0073] Table 1 shows the capacity retention rates of Examples 1 to 10 and Comparative Examples 1 to 7, and Table 2 shows the capacity retention rates of Examples 11 to 13 and Comparative Examples 2 and 3. Table 1 also shows the positive electrode active material composition, the Nb and M contents in the negative electrode, and the Nb / Ni and M / Ni ratios. Table 2 also shows the positive electrode active material composition, the Nb, M, and W contents in the negative electrode, and the Nb / Ni, M / Ni, and W / Ni ratios. The Nb, M, and W contents in the negative electrode and their ratios to Ni were determined by measuring the negative electrode taken out of the test cell after the cycle test. Furthermore, XPS confirmed that a negative electrode coating was formed on the surface of the negative electrode composite layer in all of the negative electrodes of Examples 1 to 13.

[0074] [Table 1]

[0075] [Table 2]

[0076] As shown in Table 1, the test cells of the Examples all had higher capacity retention rates after cycle testing and superior charge-discharge cycle characteristics than the test cells of the corresponding Comparative Examples (Example 1 vs. Comparative Example 1, Examples 2-6 vs. Comparative Examples 2-4, Examples 7 and 8 vs. Comparative Example 5, Example 9 vs. Comparative Example 6, and Example 10 vs. Comparative Example 7). The test cells of Examples 1 to 10 used a positive electrode active material containing Nb and M (M is at least one element selected from Ca and Sr), and a coating containing Nb and M derived from the positive electrode active material was formed on the surface of the negative electrode. On the other hand, none of the test cells of Comparative Examples 1 to 7 contained both Nb and M in the positive electrode active material, and no coating containing Nb and M was present on the surface of the negative electrode. A comparison of the results of Example 2 with those of Comparative Examples 3 and 4 reveals that the capacity retention rate is significantly improved when the positive electrode active material contains both Nb and M, compared to when the positive electrode active material contains only one of Nb and M. That is, the positive electrode active material containing predetermined amounts of Nb and M, and the negative electrode coating containing predetermined amounts of Nb and M derived from the positive electrode active material, significantly improve the charge-discharge cycle characteristics of the battery.

[0077] Furthermore, the results of Examples 11 to 13 show that even if the positive electrode active material further contains W, the effect of improving the charge-discharge cycle characteristics can be obtained. [Explanation of symbols]

[0078] 10 secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 16 outer can, 17 sealing body, 18, 19 insulating plate, 20 positive electrode lead, 21 negative electrode lead, 22 grooved portion, 23 internal terminal plate, 24 lower valve body, 25 insulating member, 26 upper valve body, 27 cap, 28 gasket

Claims

1. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, the positive electrode contains a lithium transition metal composite oxide containing Ni, Nb, M (M is at least one element selected from Ca and Sr), and Co as an optional component; In the lithium transition metal composite oxide, The content of Ni is 80 mol% or more based on the total number of moles of metal elements excluding Li, The content of Nb is 0.35 mol% or less based on the total number of moles of metal elements excluding Li, The content of M is 0.6 mol% or less based on the total number of moles of metal elements excluding Li, The content of Co is 5 mol% or less based on the total number of moles of metal elements excluding Li, the negative electrode has a negative electrode composite layer containing a negative electrode active material, and a coating containing Nb and M formed on a surface of the negative electrode composite layer, In the negative electrode, the coating on the negative electrode has an Nb content of 10 ppm to 3000 ppm with respect to the total mass of the negative electrode composite layer and the coating; a content of M contained in the coating on the negative electrode of 10 ppm to 3000 ppm with respect to the total mass of the negative electrode mixture layer and the coating.

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the lithium transition metal composite oxide contains 2 mol % or less of Co relative to the total number of moles of metal elements excluding Li, or is substantially free of Co.

3. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of Ni in the lithium transition metal composite oxide is 85 mol % or more based on the total number of moles of metal elements excluding Li.

4. 4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the coating further contains Ni.

5. In the negative electrode, a mass ratio (Nb / Ni) of the Nb content contained in the coating on the negative electrode to the Ni content contained in the coating on the negative electrode is 0.3 to 5; 5. The nonaqueous electrolyte secondary battery according to claim 4, wherein a mass ratio (M / Ni) of a content of M contained in the coating on the negative electrode to a content of Ni contained in the coating on the negative electrode is 0.3 to 10.

6. The lithium transition metal composite oxide further contains W, In the negative electrode, the coating on the negative electrode has a W content of 10 ppm to 3000 ppm with respect to the total mass of the negative electrode composite layer and the coating; 6. The nonaqueous electrolyte secondary battery according to claim 4, wherein a mass ratio (W / Ni) of a W content contained in the coating on the negative electrode to a Ni content contained in the coating on the negative electrode is 0.3 to 5.

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